1. Field of the Invention
The present invention relates to a photosensitive composition which can be used in both one-shot exposure and direct writing and, in laser direct writing, can be used irrespective of whether the wavelength of laser light is short or long, and to a dry film obtained by applying the composition onto a carrier film and then drying it.
2. Description of the Related Art
A permanent protective film that is a solder resist film is arranged on an outermost layer of a printed-wiring board. For forming this solder resist film, photoresist techniques capable of accurately forming fine patterns are widely used. In particular, photoresist techniques of alkali development type are mainly used from an environmental viewpoint, etc.
As one photoresist technique, one-shot exposure for exposing the whole surface of a dry solder resist coating on a substrate via a photomask to light from a UV lamp has been widely used (Jpn. Pat. Appln. KOKAI Publication No. 1-141904). This one-shot exposure method necessitates position adjustment between a photomask and a substrate, and this position adjustment is time-consuming. The substrate is liable to dimensional change according to heat history in circuit formation so that its dimensions deviate easily from designed dimensions. On the other hand, the photomask does not undergo dimensional change as described above and thus cannot correct the deviation in position of the substrate corresponding to its dimensional deviation. As a pattern becomes finer, the influence of dimensional deviation of the substrate becomes more significant to make accurate formation of a pattern latent image difficult. Formation of an accurate pattern latent image is very cumbersome because it is necessary that dimensions of each substrate after circuit formation are measured and a photomask corresponding thereto is prepared for each substrate. However, the one-shot exposure method has an advantage that because the whole surface of a dry solder resist coating is subjected at one time to one-shot exposure with light from a lamp, a pattern latent image can be rapidly formed.
As another photoresist technique, a direct writing method of directly writing a pattern latent image on a dry solder resist coating, on the basis of data from a computer, by using light from a lamp which generates laser light of single wavelength or UV light is also used (International Publication WO 02/096969). In this direct writing method, there are cases where h-line (405 nm) light from a blue-violet laser diode as a light source is used, light at 355 nm that is the third harmonic component from a YAG laser is used, and light (365, 405, 433 nm) from a high-pressure mercury lamp is used. In this direct writing method, a pattern latent image is formed on a substrate by scanning the substrate with light. Thus, the time for forming a pattern latent image on one substrate becomes longer than by the one-shot exposure method, and therefore productivity tends to decrease. In the direct writing method, however, the deviation of a substrate from designed dimensions as described above can be corrected with a computer by measuring the dimensions of a substrate after circuit formation, whereby a desired accurate pattern latent image can be formed.
Accordingly, both one-shot exposure process and direct writing process are carried out in parallel in production of printed-wiring boards at present. Hence, it is desired that a pattern latent image can be excellently formed by using one kind of photosensitive composition in both the one-shot exposure method and direct writing method.
A photosensitive composition for direct writing with laser is regulated to attain suitable sensitivity toward a single wavelength used, but upon exposure to UV light that is complex light, will be exposed to the light more than necessary, thus suffering from deterioration in the shape of the resulting solder resist film. On the other hand, a photosensitive composition used in the conventional one-shot exposure method has sensitivity in the range of 200 to 600 mJ, and this sensitivity range is too low to cure a thick dry solder resist coating sufficiently by light exposure with a direct imaging device. Generally, a photosensitive composition used in the one-shot exposure method attains higher sensitivity and a broader photosensitive wavelength range by a combination of a photoinitiator that absorbs UV light of relatively short wavelength (up to 370 nm) and a sensitizer that absorbs UV light of long wavelength (380 nm or more). According to the inventors' study, the initiation ability of a photoinitiator exposed directly to light is far higher than the initiation ability of a photoinitiator upon energy transfer from a sensitizer. That is, when the photosensitive composition for one-shot exposure is used, the sensitivity of the photosensitive composition tends to be made higher by irradiation with laser light of shorter wavelength than by laser light of longer wavelength. Accordingly, the sensitivity of the photosensitive composition varies depending on laser direct imaging devices that emit laser lights different in wavelength from one another, which results in a varying sectional shape of the resulting solder resist film.